Display with redundant light emitting devices

ABSTRACT

An active matrix display where in one embodiment each cell comprises: a driving circuit for providing current to light emitting devices placed in the cell under the control of a data driver signal, a first light emitting device location connected to the driving circuit and a second light emitting device location connected in series to the first light emitting device location. A first thin-film transistor (TFT) is connected in parallel with the first light emitting device location and a second TFT is connected in parallel with the second light emitting device location, its gate node connected to the gate node of the first TFT. One terminal of a third TFT is connected to the gate nodes of the first and second TFTs and selectively connects a control signal to the first and second TFTs under the control of a scan driver signal. The control signal determines which of a first or second light emitting device placed in the cell emits light when the driving circuit provides current.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of PCT Application No.PCT/US17/29418, filed Apr. 25, 2017 and United Kingdom Application No.1607248.0, filed Apr. 26, 2016, which is incorporated by reference inits entirety.

FIELD

The present invention relates to a display and a method of driving adisplay.

BACKGROUND

Displays are ubiquitous and are a core component of every wearabledevice, smart phone, tablet, laptop, desktop, TV or display system.Common display technologies today range from Liquid Crystal Displays(LCDs) to more recent Organic Light Emitting Diode (OLED) displays.

Referring now to FIG. 1, there are shown an active drive matrix for adisplay. The matrix comprises N rows of cells divided into M columns.Each cell includes a light emitting device corresponding to either: apixel for a monochrome display; or one of a red, green or blue sub-pixelwithin a color display. For color displays either: differently coloredsub-pixels can be interleaved along rows of the matrix; or respectiverows of the matrix can comprise only sub-pixels of a given colour.

A plurality of peripheral driving blocks comprise:

Scan driver—which produces pulsed signals S1 . . . Sn enablingrespective rows of the matrix to be programmed for a subsequent frame orsub-frame; and

Data driver—which delivers data outputs D1 . . . Dm to programindividual cells of a row enabled by the scan driver—these signals areupdated for each frame or sub-frame from scan line to scan line.

In some matrices, a constant supply voltage (Vdd) is provided to eachcell of the matrix to drive the light emitting device during a frameaccording to the cell programming. Typically, for a constant supplyvoltage (Vdd) implementation, the data driver provides analog outputswhich determine the brightness of a cell for a subsequent frame.

In the matrix of FIG. 1, a PWM (Pulse Width Modulation) Driver producesPWM pulses used to bias programmed cells enabling the cells to emitlight or not during a sub-frame according to their programming. (Notethat the term “PWM” is used in the present description to relate topulsed signals for activating cells within a row—such pulses may beemployed as part of a conventional PWM addressing scheme, such asdescribed in WO2010/014991 or a color sequential scheme, such asdescribed in WO2014/012247.) For PWM, the data driver typically providesdigital outputs with the PWM driver providing variable width pulseswhich in combination with the cell programming for a sub-framedetermines the brightness of a cell for a frame.

UK Patent Application No. 1604699.7 (Ref: I35-1702-01 GB) filed 21 Mar.2016 discloses a hybrid scheme where the data driver providescombinations of analog or digital outputs limiting the switchingfrequency required of the PWM driver; while UK Patent Application No.1606517.9 (Ref: I35-1702-02 GB) filed 14 Apr. 2016 discloses a cell foran active drive matrix providing voltage threshold compensation.

In FIG. 1, two synchronization blocks are employed: one located betweenthe scan driver and data driver in order to ensure that the requireddata signals are delivered after a scan pulse is applied to a row; and asecond between the data and PWM drivers to ensure that PWM pulses areapplied when data loading is completed.

Each row within the matrix is addressed with a respective scan line S1 .. . Sn which goes high or is asserted when a respective row of thedisplay is to be addressed (or programmed) by the data driver for thesubsequent frame or sub-frame. For PWM, during a given frame, for eachrow, the PWM driver provides a sequence of driving pulses usingrespective PWM signals P1 . . . Pn. Each signal P can be a time shiftedversion of the adjacent PWM signal synchronized with the scan linesignals S1 . . . Sn and data driver signals D1 . . . Dm.

Active matrix circuitry, for example, as described in WO2010/119113,uses thin film transistor technology (TFT), where cells comprisetransistors based on amorphous, oxide or polycrystalline silicontechnology manufactured on a glass substrate ranging in size from 30cm×40 cm to the latest generation (known as GEN10) of 2.88 m×3.15 m. TheTFTs are used either as voltage switches or current sources to controlthe operation of light emitting devices within each cell.

In most portable, typically battery powered, devices, the display usesthe majority of the available power. The most common user complaint forportable devices is insufficient display brightness. To extend batterylife and improve brightness levels it is necessary to develop newdisplay technologies that reduce power consumption and produce higherluminance emission from the light source.

WO2013/121051 discloses an improved light emitting device for a display,referred to as an integrated or inorganic LED (iLED) which comprises asubstrate with a semiconductor material comprising a light generatinglayer positioned on the substrate. The semiconductor material and/or thesubstrate are configured to control light internally to outputquasi-collimated light from a light emitting surface of the iLED. TheiLED comprises an optical component positioned at the light emittingsurface and configured to receive quasi-collimated light exiting thelight emitting surface and to alter one or more optical properties of atleast some of the quasi-collimated light.

Whereas OLED cells operate by passing current through organic or polymermaterials sandwiched between two glass planes to produce light; iLEDdisplays replace the OLED material with discrete LED die (which is madeof inorganic materials) placed at each cell of the display.

Standard (i.e. inorganic) LED devices have been around for many yearsand their performance (efficiency, brightness, reliability and lifetime)has been optimized over that time as the LED industry has pursued manycommercial opportunities—especially the challenge of developing LEDtechnology to enable it to replace the standard incandescent bulbs forgeneral light applications, i.e. inorganic LEDs are significantly moreefficient, bright and reliable than the new and less developed OLEDmaterials.

The concept of individually switchable standard LED dies (R, G & B) ateach pixel in a display is also known. This approach is in widespreaduse for large information displays. However, to-date it has not beenpossible to scale this approach down to smaller displays, as standardLEDs are typically planar in design and so are inefficient incontrolling the direction of emitted light. Additionally, the assemblyof the many millions of pixels needed for a laptop or smart phonedisplay is not feasible using standard assembly/manufacturingtechniques.

SUMMARY

According to a first aspect there is provided a display according toclaim 1, 8 or 16.

This aspect can provide redundancy for displays comprising discretelight emitting devices which have a high but not perfect degree ofreliability. For example, after initial pick-and-place of devices suchas iLEDs within a matrix, it is expected that their yield will be morethan 90% but less than 99% i.e. up to 10% of devices might be defective.Nonetheless, employing the present display enables only a minority oflight emitting device to be tested and known good to produce displayswith the highest level of yield.

The cell design for the display avoids the need for laser opening orshorting connections to defective devices with a display.

In some embodiments, placed light emitting devices are controlledautomatically so that only one device per cell operates.

In alternative embodiments, at least two devices are placed within eachcell and these are selectively operated so that the display can switchbetween operating modes, for example, wide-angle and narrow-angledisplay.

In a second aspect, there is provided a display according to claim 22.

In this aspect, a shared driving circuit reduces the substrate arearequired for a pixel leading to a higher pixel density. This contrastswith active drive matrices where each pixel is divided into threesub-pixels, each one representing one color (Red, Green, and Blue), witheach requiring a driving circuit to produce the necessary bias current.

According to this aspect, all light emitting devices are biased with thesame current, meaning that no more than one driving circuit per pixel isneeded. Colors are produced directly from the pixel by controlling whichlight emitting devices emit light and for how long.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 shows a conventional type active matrix display;

FIG. 2 shows a placement method for light emitting devices within adisplay according to an embodiment of the present invention;

FIG. 3 shows a cell design for an active matrix display according to anembodiment of the present invention;

FIG. 4 shows a cell design with three light emitting devices connectedin series and controlled by n-type TFTs according to another embodimentof the present invention;

FIG. 5 shows a cell design with three light emitting devices connectedin series and controlled by CMOS TFTs according to a further embodimentof the present invention;

FIG. 6 shows a cell design with three light emitting devices connectedin parallel according to a still further embodiment of the presentinvention;

FIG. 7 shows a pixel design with three differently colored lightemitting devices connected in series according to an embodiment of thepresent invention; and

FIG. 8 shows three pixels such as shown in FIG. 7 connected in seriesaccording to a still further embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Referring now to FIG. 2, when light emitting devices such as the iLEDsreferred to above are initially placed on a substrate, their yield isexpected to be between 90-95% which is lower than the acceptable yieldfor a high performance display. This means that cells containing thesedevices need to be repaired if the display is to be shipped. One methodfor doing so comprises placing a second light emitting device in a cellcontaining a defective first device and then using laser to open or meltfuses to disconnect the first device from the cell driving circuit towhich the second device is connected. The same procedure could be usedif a third or fourth device were to be placed on the cell. As will beappreciated this “physical” solution has limited applicability (5-10pixels per display), requires special equipment and increases theprocess time.

In order to overcome these problems, embodiments of the presentinvention provide a cell comprising a plurality of locations in whichlight emitting devices can be placed. Examples of cell design will bedescribed later, but we would first of all begin by describing aplacement method for a display based on a cell design capable ofreceiving three light emitting devices.

Initially, discrete light emitting devices such as iLEDs referred toabove are placed in respective first locations of all cells of thedisplay panel—this is typically performed using a pick-and-placetechnique where light emitting devices can be placed either in series orin parallel. When the pick-and-place is complete, the panel is testedthrough a visual inspection. Cells including devices that don't operateare recognized. The locations of these defective cells can be stored ina memory element available to the matrix controller (not shown) and inthis way, a map of defective cells within the display can be produced.This panel map can be used later for programming the display.

A second pick-and-place round now places devices in second locations ofcells previously identified as containing defective devices. The secondpick-and-place round can employ known good light emitting devices and itwill be seen that where the natural yield of these devices is high, thenonly a small minority of such devices need to be tested in order thatthey would be known to be good. Nonetheless, especially where each cellincludes locations for more than two devices, this testing may not benecessary.

After the second pick-and-place, another visual inspection occurs. Forany second placed device identified as working properly, the programmingof the pixel can remain as before so that the second placed device canwork according to the panel map. If any second placed device isidentified as not working, a third light emitting device can be placedin the cell. Again, these device may be known good devices and again itwill be seen that if only these devices were to be tested, only anextremely small number of such tests would need to be performed. Thelocation of the second placed devices that don't work can again bestored in memory in a second panel map which can be used for programmingthe display.

As will be appreciated, this process can be repeated for as many devicesas any potentially defective cell may be able to accommodate. Dependingon the reliability of devices being placed, placement of two, three ormore device could be necessary in order to achieve acceptable yield.

It can be understood, that as the number of devices which a cell mightbe able to accommodate is increased, more complex circuitry will berequire to control them. A maximum number of locations is determined bythe TFT process, the available pixel area and the light emitting devicemanufacture and assembly method.

Turning now to examples of cell design which can be employed in displaysbeing populated according to the method of FIG. 2.

FIG. 3 shows a cell design which can be employed within an active drivematrix and where locations for up to two placed light emitting devices,in this case ILED1 and ILED2, are connected in series. The controlcircuit comprises 3 TFTs T1 . . . T3 and a charge storage capacitance. Adata driver signal, such as D1 . . . Dm in FIG. 1 for the cell isprovided by a data driver to the cell through an otherwise conventionalvoltage threshold compensation and driving circuit. Alternatively, avoltage threshold compensation and driving circuit such as disclosed inUK Patent Application No. 1606517.9 (Ref: I35-1702-02 GB) filed 14 Apr.2016 can be employed. By comparison to a conventional 2T1C cell design,an additional control signal is needed and it is asserted (or not) inaccordance with the value in the panel map for the display. Indeed thecontrol signal state is essentially permanent as long as the value forthe cell in the panel map does not change.

If ILED1 has been checked as working normally, during the frameprogramming period when the scan driver signal, such as S1 . . . Sn inFIG. 1, is asserted, the control signal of the sub-pixel will have high“1” value, so turning ON T2 and connecting ILED1 to ground so that itwill emit light in accordance with the data driver signal.

Where ILED2 has been placed, because ILED1 has been identified asdefective, the control signal will have low “0” value meaning that T2will be turned OFF and T1 ON, so ILED1 will be shorted and ILED2 to beconnected directly to the driving circuit. Therefore, ILED1 will notemit light and only ILED 2 will do so. The truth table for thissituation is shown in Table 1.

TABLE 1 Truth Table for two ILEDs connected in series configurationControl ILED 0 ILED2 1 ILED1

In variants of the cell design of FIG. 3, a discrete storage capacitormay not be necessary, particularly for a display with a high refreshrate and where the voltage at the gates of T1 and T2 would remain stableenough without the need of the capacitance. In some implementations, adata driver signal (not shown) provided through the driving circuitmight be asserted for a complete frame. In this case, the Scan signalcould have the same timing as the data driver signal and again thecapacitance would not be needed.

As indicated above, if the desired yield can't be achieved with a celldesign capable of locating two light emitting devices, a third one canbe located. For the three devices, the complexity of the cell design isincreased.

Nonetheless, different configurations are possible depending on the TFTprocess being employed, for example, if only n-type TFTs are available(amorphous silicon TFTs and indium-Gallium-Zinc-Oxide—IGZO TFTs), oronly p-type TFTs are available (Low Temperature polycrystallineSilicon—LTPS) or if CMOS devices are available (LTPS).

Referring now to FIG. 4, there is shown a cell design implemented withn-type TFTs T1 . . . T4 only, whereas the cell design of FIG. 5 isimplemented with CMOS TFTs T1 . . . T4. A configuration with only p-typeTFTs would be the same as in FIG. 4, with all n-type TFTs being replacedwith p-type TFTs.

In both of these cases, either a constant supply voltage Vdd is providedthrough the driving circuit, or the driving circuit is switched using aPWM type pulsed signal.

As it can be seen from FIGS. 4 and 5, three control signals (A, B and C)are required as well as additional circuitry to deliver these controlsignals. Thus instead of a single bit data line as for the cell designof FIG. 3, this might comprise 3 bits. However, when implementing aredundancy scheme for the display, the relative values of these 3 bitswould remain constant through the life of the display, with each 3 bitcombination being specific to a cell in accordance with the panel mapvalues for the cell.

In each of FIGS. 4 and 5 when a defective light emitting device isplaced, it will be shorted by a corresponding TFT and at the same time afollowing functioning iLED should be connected to ground so it can emitlight. Thus, when ILED1 should emit light, it is connected to groundthrough T3, while when it is not to emit light, it is shorted by T1. Foreach configuration of TFT process, a truth table exists whichillustrates the way that the additional circuitry has to be programmed,so that a corresponding ILED will emit light. The truth tables for eachcase are shown below.

TABLE 2 Truth Table for FIG. 4 implemented with only n-type TFTs. A B CILED 0 0 0 All 0 0 1 ILED2 0 1 0 ILED1 0 1 1 None (Repair) 1 0 0 ILED3 10 1 None (Repair) 1 1 0 ILED3 1 1 1 None (Repair)

TABLE 3 Truth Table for FIG. 4 implemented with only p-type TFTs. A B CILED 0 0 0 None (Repair) 0 0 1 ILED3 0 1 0 None (Repair) 0 1 1 ILED3 1 00 None (Repair) 1 0 1 ILED1 1 1 0 ILED2 1 1 1 All

TABLE 4 Truth Table for FIG. 5 implemented with CMOS TFTs. A B C ILED 00 0 ILED2 0 0 1 All 0 1 0 None (Repair) 0 1 1 ILED3 1 0 0 None (Repair)1 0 1 ILED1 1 1 0 None (Repair) 1 1 1 ILED1

As it is be seen from the truth tables, there are states where “All”emitting devices can be biased in order to emit light. Although theremay be applications where this could be useful, it will be appreciatedthat applications which involve attempting to drive all placed devicescould cause a cell potentially containing a malfunctioning device to bedriven in an uncontrolled manner.

There are also states where no devices will emit, since all of them areshorted by the TFTs, and this can be used as a repair method. Thus, ifthe third (or last) ILED which is placed doesn't work properly, it ispreferable to treat the pixel comprising the cell as a “black” pixel andnot to emit light, rather than driving the cell in an uncontrolledmanner. Thus, in this case, the TFTs for a pixel are programmed in suchway that all devices of the pixel are shorted to ensure that they willnot emit light.

In the circuits a FIGS. 4 and 5, a scan driver signal is not shown,however, it will be appreciated that each of control signals A, B and Ccan be connected to the gates of TFTs T1 . . . T4 via respective furtherTFTs (not shown) controlled by a scan driver signal as in the circuit ofFIG. 3.

The configurations of FIGS. 3-5 have light emitting devices connected inseries to a common driving circuit and voltage threshold compensationcircuit.

FIG. 6 shows a cell design where three ILEDs are connected in parallel.The circuit of FIG. 6 can be implemented with a CMOS TFT process, as itrequires oppositely doped TFTs. In FIG. 6, if only ILED1 is placed,control signal A has low “0” value resulting in T2 being turned ON.ILED1 is directly connected to the driving circuit so it emits light.When control signal A has high “1” value, T2 is turned ON and ILED1 isshorted and then control signal B will determine which from ILED2 orILED3 will emit light. If control signal B has low value “0”, ILED2 willemit light and if control signal B has high “1” value, ILED3 willilluminate. The corresponding truth table is shown below.

TABLE 5 Truth Table for FIG. 6 implemented with CMOS TFTs. A B ILED 0 XILED1 1 0 ILED2 1 1 ILED3X: either “1” or “0”

One issue with the parallel configuration of FIG. 6, is the varyingnumber of TFTs lying between the driving circuit and the selected lightemitting device, especially the number of intermediate TFTs when ILED3should emit light. This implies that the power supply voltage has to beincreased to ensure that all TFTs operate in saturation region and thevoltage at the ILED anode is more than its threshold voltage. Also, withthe ILEDs connected in parallel and only the control signals A, B, C,one ILED will always emit light, meaning that no “Repair” configurationexists as in the implementations of FIGS. 4 and 5.

In general in the above described examples, it is desirable to driveonly a selected one light emitting device within a cell during operationof the display.

However, there are applications where it can be useful to place morethan one light emitting device within a cell and to selectively drivethese.

One such application provides a display with dynamic viewing angle.According to this application, one ILED with narrow and another ILEDwith wide beam are picked-and-placed within respective locations inevery cell of the display (or at least those whose mode is to bechanged). Depending on the required display mode, the corresponding ILEDwill be biased as explained above. For example, if only one user isviewing the display, the narrow beam (narrow viewing angle) deviceswould operate and if two or more users are viewing the display, the widebeam (wide viewing angle) would be used. (This technique could of coursebe extended to cover three different types of light emitting devicebeing placed within each cell and being selected according to therequired display mode.)

In multi-display mode case, the value of the control signal or therelative values of signals A, B, C, rather than being permanent throughthe life of the display in accordance with the panel map, is switcheddynamically according to the required display mode for the display.Also, rather than providing a per cell control signal or signals A, B,C, the control or A, B, C signals would be global, applying to the wholematrix. In a multi-display mode case where redundancy is not provided,there would be no need for a panel map.

Swapping between modes can either be user driven; or can be automatic inresponse to viewing conditions (for example, if the display controllerdetects the number of people viewing the display).

In a still further application, again two or more different sets oflight emitting devices can be placed and these can be selectively drivento provide a display which can selectively operate in one of a 2Ddisplay mode and a 3D display mode.

Referring now to FIG. 7, discrete light emitting devices, such as iLEDs,of differing colors behave differently than OLEDs in that there is asimilar limited current range where the efficiency of red, green andblue discrete light emitting devices can be high and similar.

In the cell design of FIG. 7, not only are light emitting devices of thesame color or type connected in series, but light emitting devices forsub-pixels of a cell are connected in series and driven from a commonvoltage threshold compensation and driving circuit. Since the red, greenand blue light emitting devices are connected in series, only onedriving circuit is needed, meaning that the area efficiency of the pixelon the substrate is increased so enabling high display resolution.

In FIG. 7 three identical circuits, each comprising two TFTs are appliedto each ILED. One TFT (T2, T4 and T6) is connected in parallel to eachILED and a second TFT (T1, T3 and T5) is connected to the individualcontrol signal Red Data (RD), Green Data (GD) and Blue Data (BD). Duringframe programming, a Scan driver pulse goes high “1” and T1, T3 and T5are turned ON. Whether RGB ILEDs are to be turned ON or OFF isdetermined by the value of RD, BD, GD. If RD is high “1”, T2 will beturned ON and Red ILED will be shorted resulting in not emitting light.On the other hand, if RD is low “0”, T2 will remain OFF, so the Red ILEDwill be biased with the current produced by the driving circuit and itwill emit light. The same procedure also occurs for Green ILED (T4 withGD) and Blue ILED (T6 and BD).

In the circuit of FIG. 7, the ILEDS could be turned OFF, by assertingRD, BD and GD during a frame programming period. However, this wouldmean that power would be drawn through the transistors T2, T4 and T6during this time. Thus, in the circuit of FIG. 7 an additional controlsignal Black data (BLD) controls power to the driving circuit. When BLDis asserted, no power flows through the pixel. The switching frequencyof the BLD control line would typically be much less compared to theother data lines RD, BD, GD because this will only be activated forcompletely black pixels. Furthermore, using BLD improves the contrastratio for a pixel because in this way, all current is blocked, resultingin a much deeper black colour.

As will be appreciated, not all of the iLEDs of FIG. 7 should emit lightcontemporaneously or to the same extent, in order to produce therequired grey-scale and color gamut for a frame. Ideally, the controlsignals driving the cell would be digital to ensure that each iLED wouldoperate when switched on at its optimal operating current.

Thus, displays incorporating the pixel design of FIG. 7 are eitherdriven using a Pulse-Width Modulation (PWM) or color sequential schemeof the type referenced above.

In this case, instead of a constant Vdd, PWM signals are provided from aPWM driver such as shown in FIG. 1. BLD, RD, GD and BD control signalsare asserted in accordance with the required state of a pixel for agiven sub-frame, each sub-frame varying in length. In both cases, if8-bits grey-scale is desired, the frame time is divided into 8sub-frames, each one having different duration. The Most significant Bit(MSB) of the grey-scale will have the longest pulse while the LeastSignificant Bit (LSB) will have the narrowest pulse. The differencebetween PWM and color sequence is that in the case of the PWM, all ILEDsemit light simultaneously during one sub-frame while for the colorsequence each sub-frame is further divided into Red, Green and Blueperiods meaning that for a given sub-frame duration only Red ILEDs overthe whole panel emit light, then only Green ILEDs and finally only BlueILEDs.

The difference results in the colour sequential scheme requiring atleast three times higher switching frequency since during the samesub-frame, it has to switch three times, once for each colour. Bothdriving schemes suffer from different visual artefacts that can besolved using special driving algorithms. In any case, the most importantcriterion for both schemes is that their frequency to be high enough, sothat ILED ON/OFF transition is not noticeable by the human eye.

In the circuit of FIG. 7, a maximum frequency can be achieved due to thelack of storage capacitance on all data paths; whereas in other designsa storage capacitance can be connected at the gate node of the drivingTFT in order to keep the gate voltage constant during a sub-frameduration.

In FIG. 7, the control signals determine if the parallel connected TFTsare ON or OFF. This means that their gate voltage may vary during theframe duration as long as it ensures that the TFT is turned ON. The lackof storage capacitance also reduces the data column power consumptionwhich is among the highest in a display.

On the other hand, if storage capacitance is provided for each sub-pixeland a data driver capable of providing analog or digital outputs wereemployed, then the hybrid driving scheme described in UK PatentApplication No. 1604699.7 (Ref: I35-1702-01 GB) filed 21 Mar. 2016 couldbe used to limit the switching frequency required of the PWM driver.

Finally, it will be appreciated that when a semiconductor light emittingdevice, such as an iLED emits light, there is a voltage drop between itsterminals, referred to as threshold voltage. Where light emittingdevices are connected in series, the total voltage difference betweenthe output of the driving circuit and the ground should be at least thesum of their threshold voltages, for example, in FIG. 7 the thresholdvoltages for the red, green and blue ILEDS. This means that the highsupply voltage (Vdd) (or pulsed PWM signal voltage) will be higher bycomparison to circuits driving only one light emitting device in orderto ensure all devices can emit light. Nonetheless, even though thesupply voltage Vdd will be increased in FIG. 7, the total powerconsumption will be kept the same since the total ILEDs bias current is⅓ of that for circuits driving only one light emitting device due tocurrent sharing between the light emitting devices.

Referring now to FIG. 8, in some applications, in which sufficientlyhigh supply voltages are available, more than one pixel can share thedriving circuit. Since the driving conditions of all ILEDs are the same,the driving circuit that produces the necessary bias current can beshared between more than one pixel. The only limitation for the numberof the pixels that can share the driving circuit is how high the supplyvoltage is. The condition that has to be satisfied is:

${Vdd} > {{\Delta \; V_{{driving},{circuit}}} + {\sum\limits_{i = 0}^{n}\; {Vth}_{i}}}$

where ΔV_(driving,circuit) is the voltage drop over the driving circuitwhile producing the necessary Σ_(i=0) ^(n)Vth_(i), Vth_(i) is thecombined threshold voltage of the light emitting devices within a pixeli existing on the driving circuit to ground path and n is the maximumnumber of pixels.

As mentioned above, the voltage threshold and driving circuitillustrated in FIGS. 3-8 can be any suitable design which produces anaccurate bias current with immunity to TFT threshold voltage variations.Furthermore, the driving circuit can be either dynamic meaning that itcan be programmed during every frame or sub-frame or DC (only programmedonce) or even programmed periodically with a frequency much lower thanthe frame rate (FR). In the case of the DC or periodically programmedcases, any existing current source can be used if it can be implementedin the available pixel area.

In the matrix of FIG. 1, the data driver is shown as a singular unit,however, it will be appreciated that in embodiments of the presentinvention the functionality of the data driver (and indeed the otherperipheral components) can be divided among more than one unit, forexample, with one unit providing data signals and another providingcontrol signals.

The features of the above described embodiments can be used either incombination or individually within a given display. In each case,embodiments are suitable for either wearable displays, such assmartwatches or large panel displays.

What is claimed is:
 1. A display comprising: a matrix comprising aplurality of N rows divided into a plurality of M columns of cells, eachcell being arranged to receive up to at least two light emittingdevices; a scan driver providing a plurality of N scan line signals torespective rows of the matrix, each for selecting a respective row ofthe matrix to be programmed with pixel values; and a data driverproviding a plurality of M variable level data signals to respectivecolumns of the matrix, each for programming a respective pixel within aselected row of the matrix with a pixel value; wherein each cell furthercomprises: a driving circuit for providing current to light emittingdevices placed in the cell under the control of a data driver signal, afirst light emitting device location connected to the driving circuit, afirst thin-film transistor (TFT) connected in parallel with the firstlight emitting device location, a second light emitting device locationconnected in series to the first light emitting device location, asecond TFT connected in parallel with the second light emitting devicelocation, its gate node connected to the gate node of the first TFT, anda third TFT with one terminal connected to the gate nodes of the firstand second TFTs and selectively connecting a control signal to the firstand second TFTs under the control of a scan driver signal, the controlsignal determining which of a first or second light emitting deviceplaced in the cell emit light when the driving circuit provides current.2. The display according to claim 1, wherein each cell further comprisesa capacitance connected to the gate nodes of the first and second TFTs.3. The display according to claim 1, wherein each of the first andsecond TFTs are oppositely doped.
 4. The display according to claim 1,wherein a second light emitting device is only placed in the secondlight emitting device location if a first light emitting device in thefirst light emitting device location is determined to be defective. 5.The display according to claim 1, further including a memory storingfirst light emitting device locations within the matrix containingdefective light emitting devices, the data driver being arranged toprovide control signals of opposite polarity for the locations comparedto cells with first light emitting device locations containing operatinglight emitting devices.
 6. The display according to claim 1, whereinlight emitting devices of a first type are placed in first lightemitting device locations throughout the matrix, light emitting devicesof a second type are placed in second light emitting device locationsthroughout the matrix.
 7. The display according to claim 6, wherein thecontrol signal is common to the matrix, the common control signal valuebeing chosen to drive either light emitting devices in the first lightemitting device locations or the second light emitting device locationsthroughout the matrix.
 8. A display comprising: a matrix comprising aplurality of N rows divided into a plurality of M columns of cells, eachcell being arranged to receive up to at least two light emittingdevices; and a data driver providing a plurality of M variable leveldata signals to respective columns of the matrix, each for programming arespective pixel within a selected row of the matrix with a pixel value;wherein each cell further comprises: a driving circuit for providingcurrent to light emitting devices placed in the cell under the controlof a data driver signal, a first light emitting device locationconnected to the driving circuit, a second light emitting devicelocation connected in series to the first light emitting devicelocation, a third light emitting device location connected in series tothe second light emitting device location, a first thin-film transistor(TFT) connected in parallel to the first light emitting device location,a first control signal connected to the gate node of the first TFT, asecond TFT connected in parallel to the first and second light emittingdevice locations, a second control signal connected to the gate node ofthe second TFT, a third TFT connected to a node joining the first andsecond light emitting device locations, a third control signal connectedto the gate node of the third TFT, and a fourth TFT connected inparallel to the third light emitting device location, the first controlsignal connected to the gate node of the fourth TFT, the values of thefirst, second and third control signals determining which if any of afirst, second or third light emitting device placed in the cell emitlight when the driving circuit provides current.
 9. The displayaccording to claim 8, further comprising: a scan driver providing aplurality of N scan line signals to respective rows of the matrix, eachfor selecting a respective row of the matrix to be programmed with pixelvalues; and wherein each control signal is selectively connected to thefirst to fourth TFTs through respective TFTs under the control of a scandriver signal.
 10. The display according to claim 8, wherein the TFTsare one of either: n-type or p-type.
 11. The display according to claim8, wherein a second light emitting device is only placed in the secondlight emitting device location if a first light emitting device in thefirst light emitting device location is determined to be defective. 12.The display according to claim 11, wherein a third light emitting deviceis only placed in the third light emitting device location if a secondlight emitting device in the second light emitting device location isdetermined to be defective.
 13. The display according to claim 8,further including a memory storing first, second or third light emittingdevice locations within the matrix containing defective light emittingdevices, the data driver being arranged to provide relatively valuedcontrol signals to each cell in accordance with the defective state oflight emitting devices placed in the cell.
 14. The display according toclaim 8, wherein light emitting devices of a first type are placed infirst light emitting device locations throughout the matrix, lightemitting devices of a second type are placed in second light emittingdevice locations throughout the matrix and light emitting devices of athird type are placed in third light emitting device locationsthroughout the matrix.
 15. The display according to claim 14, whereinthe control signals are common to the matrix, the common control signalvalues being chosen to drive light emitting devices in either the first,second or third light emitting device locations throughout the matrix.16. A display comprising: a matrix comprising a plurality of N rowsdivided into a plurality of M columns of cells, each cell being arrangedto receive up to at least two light emitting devices; a scan driverproviding a plurality of N scan line signals to respective rows of thematrix, each for selecting a respective row of the matrix to beprogrammed with pixel values; and a data driver providing a plurality ofM variable level data signals to respective columns of the matrix, eachfor programming a respective pixel within a selected row of the matrixwith a pixel value; wherein each cell further comprises: a drivingcircuit for providing current to light emitting devices placed in thecell under the control of a data driver signal, a first light emittingdevice location connected to the driving circuit via a first thin-filmtransistor (TFT), a second light emitting device location connected tothe driving circuit via second and third series connected TFTs, a thirdlight emitting device location connected to the driving circuit via afourth TFT connected in series with the third TFT, the second TFT beingconnected in parallel to the third TFT and the first TFT being connectedin parallel to the second to fourth TFTs, a fifth TFT with one terminalconnected to the gate nodes of the second and third TFTs and selectivelyconnecting a first control signal to the first and second TFTs under thecontrol of a scan driver signal, and a sixth TFT with one terminalconnected to the gate node of first TFT and selectively connecting asecond control signal to the first and second TFTs under the control ofa scan driver signal, the control signals determining which of a first,second or third light emitting device placed in the cell emit light whenthe driving circuit provides current.
 17. The display according to claim16, wherein each of second and third TFTs are oppositely doped.
 18. Thedisplay according to claim 6, wherein light emitting devices of thefirst type emit light with a narrow beam and light emitting devices ofthe second type emit a wider beam.
 19. The display according to claim 1,wherein the light emitting devices comprise discrete light emittingdiodes.
 20. A method of populating a display comprising: placing firstdiscrete light emitting devices at first light emitting device locationswithin a matrix, the matrix including a plurality of cells each beingarranged to receive at least two light emitting devices; testing thedisplay to determine one or more first cells containing a defectivefirst light emitting device; and placing second discrete light emittingdevices at second light emitting device locations within the one or morefirst cells determined to contain a defective first light emittingdevice.
 21. The method according to claim 20, further comprising:subsequent to placing the second discrete light emitting devices at thesecond light emitting device locations, testing the display to determineone or more second cells containing two defective light emittingdevices; and storing locations of the one or more first cells and one ormore second cells in a memory.
 22. A display comprising: a matrixcomprising a plurality of N rows divided into a plurality of M columnsof cells; a scan driver providing a plurality of N scan line signals torespective rows of the matrix, each for selecting a respective row ofthe matrix to be programmed with pixel values; and a data driverproviding a plurality of M variable level data signals to respectivecolumns of the matrix, each for programming a respective pixel within aselected row of the matrix with a pixel value; wherein each cell furthercomprises: a driving circuit for providing current to light emittingdevices placed in the cell, and at least one pixel comprising: a firstlight emitting device connected to the driving circuit for emittinglight of a first color, a first thin-film transistor (TFT) connected inparallel to the first light emitting device, a first data driver signalselectively connected to the gate node of the first TFT under thecontrol of a scan driver signal, a second light emitting deviceconnected in series to the first light emitting device for emittinglight of a second color, a second TFT connected in parallel to the firstlight emitting device, a second data driver signal selectively connectedto the gate node of the second TFT under the control of a scan driversignal, a third light emitting device connected in series to the secondlight emitting device for emitting light of a third color, a third TFTconnected in parallel to the third light emitting device, and a thirddata driver signal selectively connected to the gate node of the thirdTFT under the control of a scan driver signal, the values of the first,second and third data driver signals determining which if any of thefirst, second or third light emitting devices emit light when thedriving circuit provides current.
 23. The display according to claim 22,wherein the driving circuit is selectively connected to a power supplyunder the control of a fourth data driver signal, the fourth data driversignal determining if any of the first to third light emitting devicesshould emit light during a frame.
 24. The display according to claim 22,wherein each cell comprises up to n series connected pixels, where:Vdd>ΔV_(driving,circuit)+Σ_(i=0) ^(n)Vth_(i) where Vdd is a supplyvoltage for the driving circuit, ΔV_(driving,circuit) is the voltagedrop over the driving circuit while producing the necessary, Σ_(i=0)^(n)Vth_(i) where Vth_(i) is the combined threshold voltage of the lightemitting devices within a pixel i connected to the driving circuit. 25.The display according to claim 22, wherein the driving circuit providespulses of current to a cell, each pulse corresponding to a sub-frame foran image.